How do I switch to the active tab in Selenium?

We developed a Chrome extension, and I want to test our extension with Selenium. I created a test, but the problem is that our extension opens a new tab when it's installed, and I think I get an exception from the other tab. Is it possible to switch to the active tab I'm testing? Or another option is to start with the extension disabled, then login to our website and only then enable the extension. Is it possible? Here is my code: